This application claims priority under 35 U.S.C. §119(a) on Japanese Patent Application No. 2007-304774 filed in Japan on Nov. 26, 2007, the entire contents of which are hereby incorporated by reference.
The present invention relates to a double-sided wiring board provided with a connecting device serving as a connecting region to be connected to the outside, a manufacturing method of such a double-sided wiring board, and a mounting double-sided wiring board using such a double-sided wiring board.
In recent years, portable electronic equipment such as mobile phones has been widely provided. Although the portable electronic equipment has been made more compact in view of the portability, even greater functionality is sought. For this reason, there is an extreme demand for, for example, higher-precision, smaller, and thinner circuit wiring boards used in the portable electronic equipment.
In such circumstances, the circuit wiring boards are required to provide high-density wiring, high-functionality wiring (solid wiring or bending wiring), and high-density mounting. Thus, there also is an extreme demand for a higher-precision and higher-density interlayer connection.
FIGS. 20A and 20B are explanatory diagrams schematically showing the structure of a double-sided wiring board according to Conventional Example 1, where FIG. 20A is a plan view and FIG. 20B is an end face view of a cut section as viewed from arrows B-B in FIG. 20A. It should be noted that hatching is completely omitted from the end face view for clarity of illustration. This also applies to end face views described below.
In a double-sided wiring board 101 according to Conventional Example 1, a through-hole 112 is formed in an insulating substrate 110; a first-side conductive layer 121 and a second-side conductive layer 122 are connected to each other by a connecting conductive layer 130; a connecting device 101cd having a so-called through-hole, mini-land through-hole, landless through-hole, or the like is thus formed; and the connecting device 101cd is connected from the outside.
In the double-sided wiring board 101 according to Conventional Example 1, the connecting conductive layer 130 is made small in order to meet the demand for higher precision and higher density, and thus there is a problem with the strength.
FIGS. 21A and 21B are explanatory diagrams schematically showing the structure of a double-sided wiring board according to Conventional Example 2, where FIG. 21A is a plan view and FIG. 21B is an end face view of a cut section as viewed from arrows B-B in FIG. 21A.
In a double-sided wiring board 101 according to Conventional Example 2, a substrate hole 111 is formed in an insulating substrate 110; a first-side conductive layer 121 and a second-side conductive layer 122 in the form of connecting lands, which replace the through-hole 112 of Conventional Example 1, are connected by a connecting conductive layer 131; a connecting device 101cd having a so-called laser via (mini-land laser via, landless laser via) or the like is thus formed; and the connecting device 101cd is connected from the outside.
In the double-sided wiring board 101 according to Conventional Example 2, a first-side connecting land portion 161 and a second-side connecting land portion 162 are formed, so that the strength is improved compared to Conventional Example 1. However, portions where a connection is made to the insulating substrate 110 are very small, and thus it is difficult to obtain sufficient strength.
FIGS. 22A and 22B are explanatory diagrams schematically showing the structure of a mounting double-sided wiring board according to Conventional Example 3, where FIG. 22A is a plan view and FIG. 22B is an end face view of a cut section as viewed from arrows B-B in FIG. 22A.
In a mounting double-sided wiring board 102 according to Conventional Example 3, a solder ball 10b, which is to be connected to the outside, is formed on the double-sided wiring board 101 shown as Conventional Example 2. However, since the first-side connecting land portion 161 is in the form of a recess, an air bubble Air is in some cases trapped in the solder ball 102b joined thereto, and thus there is a problem with the reliability of connection.
The circuit wiring boards shown in Conventional Examples 1, 2, and 3 are used for various purposes. Particularly when the circuit wiring boards are used in the above-mentioned portable electronic equipment, there is a considerable demand for thinner and flexible double-sided wiring boards 101. That is to say, the insulating substrate 110, the first-side conductive layer 121, and the second-side conductive layer 122 are required to be made thinner and flexible. Therefore, the insulating substrate 110, the first-side conductive layer 121, and the second-side conductive layer 122 are each formed to be thin, which results in a decrease in the mechanical strength.
With the decrease in the mechanical strength of each of the insulating substrate 110, the first-side conductive layer 121, and the second-side conductive layer 122, the adhesion of the conductive layers (the first-side conductive layer 121 and the second-side conductive layer 122) to the insulating substrate (the insulating substrate 110) is decreased, and thus there is the problem that the peel strength of the conductive layers is decreased. Moreover, as the patterns (the connecting land portions) are made smaller and finer, the bonding area of the conductive layers with respect to the insulating substrate is reduced, which also results in a decrease in the peel strength of the conductive layers.
In the above-described circumstances, the peel strength of the connecting device, which is formed on a double-sided wiring board and serves as a connecting region (a connecting terminal) to be connected to the outside, is also decreased, and thus there is the problem that the reliability of connection in a mounting double-sided wiring board in which a solder ball is mounted on a double-sided wiring board is also decreased.
Moreover, as the connecting land portions become smaller, solder joining becomes difficult, with the result that there also is the problem of a decrease in the joining strength. Furthermore, as shown in Conventional Example 3, there is the problem of air bubble entrapment.
Moreover, the improvement of the bond strength between the conductive layers on both sides of a double-sided wiring board is disclosed in, for example, three documents: JP 2002-57429A, JP 2004-281437A, and JP 2007-189125A.